The overall objective of the proposed pilot research is to define functional and neural mechanisms critical for the perception, and in particular, the differentiation of speech sounds. Within this general objective are 3 specific aims: 1) to characterize differences between processes underlying the discrimination and identification of speech sounds; 2) to delineate processes responsible for the differentiation of speech sounds by: voicing, place-of-articulation, vocalic environment, and syllabic position; and 3) to identify the neuroanatomic structures associated with the processes defined by Specific Aims #1 and #2. These goals will be accomplished by a combination of behavioral testing and neuroanatomic localization through electrical interference applied directly to the cortex of fully conscious patients who require long-term placement of subdural electrode grids for clinical purposes, but whose language and cognitive functions are otherwise normal, as assessed by standardized tests. These patients will be studied at baseline (no electrical interference) and during electrical interference at specific cortical sites made possible by the electrode grid. Normal subjects, matched to the patient population, will participate in the study as baseline controls, but will not undergo electrical interference testing. Stimuli will comprise digitized, natural speech tokens contrasted by type and number of acoustic and contextual cues. Experimental tasks will incorporate traditional auditory discrimination and identification paradigms as well as evaluations of speech comprehension and production. Within-subject comparisons between baseline and electrical interference data will be performed at each cortical site tested. Between-group comparisons of patients' baseline performance and that of matched normal subjects will provide an independent evaluation of non-interference performance. Preliminary data have established the feasibility and utility of the procedures proposed and have tentatively identified functionally and neuroanatomically independent perceptual mechanisms. This pilot project should provide precise functional and neuroanatomic characterizations of the processes involved in the discrimination and identification of speech sounds. The results of these studies will, therefore, have implications for understanding the functional and neural bases of speech perception, and will provide a foundation for improved diagnosis and treatment of speech perception disorders.